1. Field of the Invention
The present invention relates to a fixing apparatus mounted to an image forming apparatus.
2. Related Background Art
An example of a conventional image forming apparatus will now be described with reference to an electro-photographic printer.
In the electro-photographic printer, an image forming operation is performed as follows. First of all, a surface of a photosensitive member having a photosensitive layer is uniformly electrified or charged and then, the photosensitive member is exposed in accordance with an image signal sent from a host computer, thereby forming a latent image. Then, after the latent image is developed as a visual image by developer (toner), the visual image (toner image) is transferred onto a recording material, and then, the toner image together with the recording material is passed through a fixing apparatus to thermally fix the toner image, thereby forming a fixed image. In general, some of fixing apparatuses includes a heater as a heat source, a rotary member heated by the heater, a pressure member that contacts with the rotary member to form a nip portion therebetween, a temperature detection portion for detecting a temperature of the heater, and control means for controlling the electrifying to the heater.
In the fixing apparatus, the temperature of the rotary member must be increased to heat the recording material adequately, thereby preparing a fixing operation. More specifically, after the temperature of the rotary member is increased up to a predetermined temperature, the fixing operation is carried out. Hereinafter, an operation by which the temperature of the rotary member is previously increased to heat the recording material is referred to as a pre-heating operation. In the fixing apparatus in which the pre-heating operation is performed, it is desirable that the fact that the temperature of the rotary member reaches the predetermined temperature (target temperature) utilizes a condition for the ending of the pre-heating operation, i.e. the starting of the image formation.
On the assumption that the pre-heat is performed from a low temperature condition, it is desirable to set the target temperature to a higher value, supposing that the heat is dispersed from the fixing apparatus. However, in a case where the temperature around the fixing apparatus is high, if the target temperature is set to the higher value, excessive heat would be supplied, thereby arising problems that an image problem such as hot offset is generated and/or that the pre-heating operation time is extended excessively.
Japanese Patent Application Laid-open No. H10-26901 (1998) discloses an arrangement in which, to cope with the variation of a surrounding environment of the image forming apparatus, on the basis of the temperature of the rotary member prior to the heating (referred to as “initial temperature” hereinafter), the target temperature is set to a low value if the initial temperature is high and the target temperature is set to a high value if the initial temperature is low.
However, in the arrangement disclosed in the Japanese Patent Application Laid-open No. H10-26901, when the fixing operation is performed after the pre-heating operation is finished, since a temperature of a central region of the rotary member reaches a temperature enough to heat the recording material adequately but temperatures of end-portions of the rotary member do not reach such a temperature, poor fixing may be generated at end-portions of the recording material. Further, when the temperatures of the end-portion regions of the rotary member reach the temperature enough to heat the recording material adequately, the temperature of the central region of the rotary member is increased excessively, which may cause the hot offset.
Japanese Patent Application Laid-open No. 2002-174989 discloses a fixing apparatus comprising a central region heater for heating a central region of a rotary member, an end-portion region heater for heating an end-portion region of the rotary member, a main-thermistor for detecting a temperature of the central region of the rotary member, a sub-thermistor for detecting a temperature of the end-portion region of the rotary member and wherein the electrifying to the central region heater is controlled on the basis of a detected temperature of the main-thermistor and the electrifying to the end-portion region heater is controlled on the basis of detected temperature of the sub-thermistor. When the central region heater and the end-portion region heater are heated so that the main-thermistor and the sub-thermistor reach target temperatures respectively, if a difference between the temperature detected by the main-thermistor and the temperature detected by the sub-thermistor exceeds a predetermined temperature difference, the temperature of the heater heating the higher temperature region is further increased and the temperature of the heater heating the lower temperature region is decreased, thereby making longitudinal temperature distribution of the rotary member uniform.
However, in the arrangement disclosed in the Japanese Patent Application Laid-open No. 2002-174989, although the uniformity during the fixing operation can be achieved by providing the plurality of heaters and thermistors, in this arrangement, also in the pre-heating operation, it is designed that the main-thermistor and the sub-thermistor reach the target temperature regardless of the initial temperature. Thus, in the arrangement disclosed in the Japanese Patent Application Laid-open No. 2002-174989, since the target temperatures of the main-thermistor and the sub-thermistor are set to the same temperature until the difference between the temperature detected by the main-thermistor and the temperature detected by the sub-thermistor reaches the predetermined temperature difference, the pre-heating operation time may extended excessively.
The present invention is made in consideration of the above-mentioned circumstances and aims to provide a fixing apparatus in which a pre-heating operation time is not extended excessively, while maintaining longitudinal temperature distribution of a rotary member to temperature distribution which does not cause poor fixing and/or hot offset when a fixing operation is carried out after a pre-heating operation is finished regardless of an initial temperature of the rotary member.